Thermometer calibration over time: Are the data correct?

A Jell-O thermometer. (Wikimedia Commons)

Whenever I read a story about temperature changes since the 19th century, I wonder about the source of those decades of data. Did the thermometers of 1900 have the fidelity of those of today? Were they calibrated the same? Because we’re talking about a 1-degree difference, sometimes less — and that’s somewhere in the neighborhood of 1 percent in the range of possible temperature readings. Add to that changing placement of instruments over the years and increasing ambient heat from built environments. In the end, can we trust the data?

I am not a climate-change skeptic, but I do wonder if the data of the past is truly comparable to that of the present.